A rare, mostly benign, primary tumor of the meninges (arachnoid cap cells), usually located in the supratentorial compartment, commonly appearing in the sixth and seventh decade of life, clinically silent in most cases or causing hyperostosis close to the tumor and resulting in focal bulging and localized pain in less than 10% of cases. Additional features may include headache, seizures, gradual personality changes (apathy and dementia), anosmia, impaired vision, exophthalmos, hearing loss, ataxia, dysmetria, hypotonia, nystagmus, and rarely spontaneous bleeding.

Inability to recognize objects not because of sensory deficit but because of the inability to combine components of sensory impressions into a complete pattern. Thus, agnosia is a neurological condition which results in an inability to know, to name, to identify, and to extract meaning from visual, auditory, or tactile impressions.